Automotive and other durable coatings are complex multilayer systems which rely on certain performance criteria at each level for overall success. The bulk of the responsibility for providing improved chip resistance lies with the primer surface layer. However, improving chip resistance properties of automotive coatings or other types of protective or decorative durable coatings is clearly a system-dependent task. For many systems, there appears to be several key ingredients to achieving incremental improvements. A chip-resistance test is by nature high in stress, impact, and shear, which intuitively generates hypotheses based on the softness, or rubber-like characteristics of the primer layer. Any contributions from the primer to increase brittleness eventually may lead to reduced chip performance. This can be related to the degree of cross linking. Very low and very high levels of cross linking in such systems lead to poor chip resistance. In the case of low cross linking, the coating does not possess the strength properties necessary to withstand the impact and shear forces applied by the gravel. The base molecular weight is usually too low to provide appropriate elastic properties associated with light cross linking. The result is flow and scission on gravel impact. An extremely high cross link density affords large numbers of covalent cross links which impart brittleness to the coating, and failure is based on the inability of the coating to absorb and evenly distribute these concentrated forces applied at the gravel impact. The material neither flows, nor elastically deforms, but instead suffers brittle fracture. Further, cross linkers often cause VOCs by virtue of VOC by product release from the cross link reaction between the cross linkers and polymeric resin. Also complicating systems which use separate cross linking agents is the propensity for the cross linking agent to react with itself. For example, self-condensation of melamine cross linkers is a potential competing reaction, which may result in local areas of high concentration of highly cross linked melamine resin. This would produce localized brittleness which may result in chip or adhesion failures. Another key contributor to successful chip resistance is the adhesion of the primer to the substrate, and the subsequent adhesion of another paint coating to the primer (collectively intercoat adhesion). A major contributor to this effect is the functionality of the primer resin-polar group functionality including hydroxyl and carboxyl that play a key role in interactions with the substrate and other paint layers.
In order to obtain the optimal chip-resistant properties, the coatings overlying the primer and the primer base resin must each deliver a balance of properties to the final coating. The easiest way to maximize mechanical properties of the primer resin is to increase molecular weight. Increases in molecular weight, however, generally increase the viscosity of such resins. While high viscosity resins may be cut with organic solvents, this results in an undesirable increase in volatile organic compounds (VOCs).